I don't wear one. I wore one about a decade ago for about a year and a half. The funny thing is - most of the concussions I've got was during my time of wearing a helmet... and none of my concussions were a result of hitting my skull - they were all caused by the teeth smashing together.
either way - stopped wearing a helmet (as I said) almost a decade ago. I could start giving my "excuses" as to why I don't wear a helmet, but you'll all just say I'm stupid, so there's no point. bottom line is: I like the feel, freedom, and visibility from not wearing one.
I just always keep in mind that I am not wearing a helmet, so when I'm falling I don't let my head smash against the ground, and if I'm falling in the trees, my only concern is to make sure I don't fall head first into shit. I'm not worried about hitting my head and dying in the park - it's out on the mountain in the trees with cliffs and chutes - trees and rocks will fuck you up!
Oh yea - and if you're using a helmet to give yourself more confidence - you'll probably end up fucking yourself up, so don't.